EFFECT OF ELONGATION ON THE DISLOCATION STRUCTURE AND CRACK INITIATION OF HARDENED AND TEMPERED 40Mn STEEL.

The process of deformation and crack initiation of high temper (550 degree C) medium-carbon steel 40Mn has been investigated using smooth tensile specimens. The midgage length part of the specimen was spark-cut into 0. 3mm thick sheets. The direct transmission observation of thin foil specimens was carried out on TEM. The evolution of the dislocation structure with elongation in hardened and tempered steel was studied. In the lath ferrite, the feature of the dislocation structure is an irregular network. At the stage of comparatively small elongation ( delta equals 0. 2-2. 7%), near the interface of ferrites and the ferrite-cementite interface, regular dislocation networks appear and the dislocation density increases. At the deformation stage of delta equals 4. 7%, the dislocations tangle and densify. The crack nucleation in hardened and tempered steel has been observed by SEM and optical microscope.